The present invention relates to a station line interface circuit for connection a subscriber station, such as a telephone, to a telecommunications network, such as a private automatic branch exchange ("PABX").
Station line interface circuits, also known as "subscriber line interface circuits", are well known in the art. The basic purpose of a this type of circuit is to convert two wire balanced voice signals to either two or four wire unbalanced signals. Stated another way, the station line interface circuit makes the conversion between a station loop circuit (comprising, e.g. a tip lead, a telephone instrument and ring lead) and a two-way (transmit and receive) voice path of a telecommunications network to which other telephones, trunk circuits, announcement recordings or tone sources may be connected.
Ancillary functions of a station line interface circuit include electronically limiting the amount of current that is supplied to the station loop, monitoring the loop so as to determine whether a telephone in the loop circuit is on or off hook, injecting ringing voltage into the loop circuit to alert a telephone user to an incoming call, protecting against overvoltage conditions which may be imposed on the loop by accidental connection to power lines or by nearby lightning strikes, and selectively denying current to the loop upon receipt of an external command.
The transition between a two-wire telephone loop and four-wire (transmit and receive) transmission paths is effected by a so-called "hybrid" circuit. The U.S. Pat. Nos. 4,178,485; 4,281,219 and 4,485,277 disclose transformerless hybrid circuits which serve this function. However, all of the ancillary functions indicated above have not been provided for in these known hybrid circuits.
One of the principal disadvantages of these known circuits is that the circuit impedance appearing across the tip and ring leads remains constant no matter how long or short the wire telephone lines may be. If the telephone lines are extremely long, the loop resistance (wire resistance plus the resistance of the telephone instrument) will be high and, consequently, the current delivered to the instrument will be low. Conversely, if the two-wire pair is extremely short, excessive current will be delivered to the telephone instrument, creating heat and wasting power.
It is desirable, therefore, to provide a station line interface circuit which appears as a voltage source when the two-wire lines connected to it are extremely long and appears as a current source when these lines are short.